In conventional pneumatic tires, normally, the height of each block is fixed (see FIG. 19).
The block 100 deforms (at the time of running) as shown in FIG. 20B, and a ground contact pressure of a tread surface 102 becomes ununiform (that is, the ground contact pressure becomes higher at ends of the block; see FIG. 20A). Therefore, it becomes difficult that braking force or driving force be transmitted from the entire tread surface 100 to a road surface 104.
A portion of the block 100 is worn down in an early stage due to the ununiformity of ground contact pressure, that is, uneven wear is apt to occur. Further, when shear stress caused by local concentration of ground contact pressure is input, only a region in the vicinity of a ground contact end of the block 100 at an input side of shear stress contacts the road surface locally in high pressure, and the tread surface 102 is turned up (see FIG. 21). As a result, the handling stability of tires is adversely affected.
In order to improve ground contact characteristics of tires, improvements of a tread pattern and the like have conventionally been made. However, under the existing circumstances, there is a limit to the improvement because the aspect of water drainability or the relationship with other various characteristics must be considered simultaneously.
Further, there has also been made an improvement in which a region in the vicinity of the ground contact end of the block is chamfered for the purpose of making the ground contact pressure uniform. For example, it is general that an end 106 in which the ground contact pressure concentrates be tapered as shown in FIG. 22 or be chamfered substantially in an R-shaped manner as shown in FIG. 23.
In order that the ground contact pressure be made uniform by chamfering the block in an R-shaped manner, it is not necessary that the R-shaped curved surface be tangent to a groove surface 108 at the ground contact end of the block.
Accordingly, it is known that chamfering in which an R-shaped curved surface only tangent to the ground contact surface is provided as shown in FIG. 24, is very effective.
However, since a single curvature is used in the above-described chamfers, the effect of equalizing the ground contact pressure is not sufficiently obtained, although the ground contact pressure at the end of the block is reduced.
It is necessary for the purpose of further improvement in handling stability that the shape of the chamfer faithfully corresponds to the ground contact pressure distribution.
Further, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 62-279105, a technique has been proposed in which a convex (chamfered) shape is formed at an upper portion of a block and along a circumferential or transverse direction of a tire. However, it is hard to obtain a tread surface of the block in which the above-described effect consists with other various characteristics only by using such technique. Furthermore, normally, determination of the shape of the tread surface is based on trial and error, and is also attended with difficulties.
The reason why is described below.
First, the distribution of ground contact pressure depends on the configuration of a tread pattern and is affected by deformation of the block to which input is applied (for example, deformation from the state of FIG. 19 to that of FIG. 20B, or deformation from the state of FIG. 19 to that of FIG. 21). Therefore, it is difficult to predict the distribution.
Further, there are various inputs applied to tires, and it is therefore very difficult to uniquely determine an aim for improvement suitable for all of the inputs.
Accordingly, a technique is demanded in which a method for changing a distribution of chamfer on the surface of the block is uniquely defined.
It is therefore an object of the present invention to provide a pneumatic tire in which ununiformity of ground contact pressure, is prevented by defining a proper height of each block on the tread pattern, and handling stability and uneven wear resistance are